Improve documentation (add reference of HttpClientCustomizer)

Would be great if you could add a mention in the documentation about  the existence of the class HttpClientCustomizer

**Additional context**
I was looking a way to decorate the the httpclient and I couldn't find a clean way until I open the code ( HttpClientFactory) and there I found the customizer  is a great feature and I think would be awesome let people know it through the documentation 